a bhikkhu is a fully ordained male buddhist monastic while female monastics are called bhikkhunis. bhikkhus and bhikkhunis keep many precepts: they live by the vinaya's framework of monastic discipline, the basic rules of which are called the patimokkha. their lifestyle is shaped so as to support their spiritual practice, to live a simple and meditative life, and attain nirvana.

the special dress of ordained people, the robes, comes from the idea of wearing cheap clothes just to protect the body from weather and climate. They shall not be made from one piece of cloth, but mended together from several pieces. Since dark red was the cheapest colour in Kashmir, the Tibetan tradition has red robes. In the south, yellow played the same role, though the color of saffron also had cultural associations in India; in East Asia, robes are yellow, grey or black.

*photos taken from vesak parade in candi mendut

More information from http://en.wikipedia.org/wiki/Bhikkhu